Sheet production apparatus for removing a crystalline sheet from the surface of a melt using gas jets located above and below the crystalline sheet

ABSTRACT

In one embodiment, a sheet production apparatus comprises a vessel configured to hold a melt of a material. A cooling plate is disposed proximate the melt and is configured to form a sheet of the material on the melt. A first gas jet is configured to direct a gas toward an edge of the vessel. A sheet of a material is translated horizontally on a surface of the melt and the sheet is removed from the melt. The first gas jet may be directed at the meniscus and may stabilize this meniscus or increase local pressure within the meniscus.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to the U.S. Patent Application entitled“Removing a Sheet from the Surface of a Melt Using Gas Jets,” filed Mar.1, 2011 and assigned U.S. application Ser. No. 13/037,789 andprovisional patent application entitled “Removing a Horizontal CrystalSheet from the Surface of a Melt Using Gas Jets,” filed May 6, 2010 andassigned U.S. App. No. 61/332,073, the disclosure of which is herebyincorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of contractnumber DE-EE0000595 awarded by the U.S. Department of Energy.

FIELD

This invention relates to sheet formation from a melt and, moreparticularly, to removing the sheet from the melt.

BACKGROUND

Silicon wafers or sheets may be used in, for example, the integratedcircuit or solar cell industry. Demand for solar cells continues toincrease as the demand for renewable energy sources increases. Themajority of solar cells are made from silicon wafers, such as singlecrystal silicon wafers. Currently, a major cost of a crystalline siliconsolar cell is the wafer on which the solar cell is made. The efficiencyof the solar cell, or the amount of power produced under standardillumination, is limited, in part, by the quality of this wafer. As thedemand for solar cells increases, one goal of the solar cell industry isto lower the cost/power ratio. Any reduction in the cost ofmanufacturing a wafer without decreasing quality will lower thecost/power ratio and enable the wider availability of this clean energytechnology.

The highest efficiency silicon solar cells may have an efficiency ofgreater than 20%. These are made using electronics-grade monocrystallinesilicon wafers. Such wafers may be made by sawing thin slices from amonocrystalline silicon cylindrical boule grown using the Czochralskimethod. These slices may be less than 200 μm thick. To maintain singlecrystal growth, the boule must be grown slowly, such as less than 10μm/s, from a crucible containing a melt. The subsequent sawing processleads to approximately 200 μm of kerf loss, or loss due to the width ofa saw blade, per wafer. The cylindrical boule or wafer also may need tobe squared off to make a square solar cell. Both the squaring and kerflosses lead to material waste and increased material costs. As solarcells become thinner, the percent of silicon waste per cut increases.Limits to ingot slicing technology may hinder the ability to obtainthinner solar cells.

Other solar cells are made using wafers sawed from polycrystallinesilicon ingots. Polycrystalline silicon ingots may be grown faster thanmonocrystalline silicon. However, the quality of the resulting wafers islower because there are more defects and grain boundaries and this lowerquality results in lower efficiency solar cells. The sawing process fora polycrystalline silicon ingot is as inefficient as a monocrystallinesilicon ingot or boule.

Yet another solution is to pull a thin ribbon of silicon vertically froma melt and then allow the pulled silicon to cool and solidify into asheet. The pull rate of this method may be limited to less thanapproximately 18 mm/minute. The removed latent heat during cooling andsolidifying of the silicon must be removed along the vertical ribbon.This results in a large temperature gradient along the ribbon. Thistemperature gradient stresses the crystalline silicon ribbon and mayresult in poor quality multi-grain silicon. The width and thickness ofthe ribbon also may be limited due to this temperature gradient. Forexample, the width may be limited to less than 80 mm and the thicknessmay be limited to 180 μm.

Producing sheets horizontally from a melt may be less expensive thansilicon sliced from an ingot and may eliminate kerf loss or loss due tosquaring. Sheets produced horizontally from a melt also may be lessexpensive than a silicon ribbon pulled vertically from a melt.Furthermore, sheets produced horizontally from a melt may improve thecrystal quality of the sheet compared to silicon ribbons pulledvertically or at an angle from a melt. A crystal growth method such asthis that can reduce material costs would be a major enabling step toreduce the cost of silicon solar cells.

Horizontal ribbons of silicon that are physically pulled from a melthave been tested. In one method, a seed attached to a rod is insertedinto the melt and the rod and resulting sheet are pulled at a low angleover the edge of the crucible. The angle, surface tension, and meltlevel are balanced to prevent the melt from spilling over the crucible.It is difficult, however, to initiate and control such a pullingprocess. First, the angle of inclination adjustment to balance gravityand surface tension of the meniscus formed at the crucible edge may bedifficult. Second, a temperature gradient along the ribbon at theseparation point between the sheet and the melt may cause dislocationsin the crystal if the cooling plate is near this separation point.Third, inclining the sheet above the melt may result in stress at thefreeze tip. This freeze tip may be where the sheet is thinnest and mostfragile so dislocations or breaks in the sheet may occur. Fourth, acomplex pulling apparatus may be needed to obtain the low angle.

The sheet must be removed from the melt surface without spilling themelt. Thus, the meniscus between the underside of the sheet and the meltmust remain stable or attached to the vessel. Previously, pressure hasbeen reduced in the melt side of the meniscus to maintain the stabilityof the meniscus. In one example, the Low Angle Silicon Sheet (LASS)method inclined the sheet at a small angle and pulled up on the melt.This created negative pressure in the melt relative to atmosphericpressure and provided a pressure across the meniscus. In anotherexample, the melt may be flowed over the edge of a spillway. The drop influid in the nape of the spillway provides a negative pressure in themelt to stabilize the meniscus. However, there is a need in the art foran improved method of removing a sheet from a melt and, moreparticularly, removing a sheet from a melt with local pressure.

SUMMARY

According to a first aspect of the invention, a sheet productionapparatus is provided. The apparatus comprises a vessel configured tohold a melt of a material. A cooling plate is disposed proximate themelt and is configured to form a sheet of the material that ishorizontal on the melt proximate the cooling plate. A first gas jet isconfigured to direct a gas toward an edge of the vessel.

According to a second aspect of the invention, a method of sheetproduction is provided. The method comprises translating a sheet of amaterial horizontally on a surface of a melt of the material. A gas isdirected from a first gas jet at a meniscus of the melt and the sheet isremoved from the melt.

According to a third aspect of the invention, a method of forming asheet is provided. The method comprises applying a seed to a melt of amaterial. A gas is directed from a first gas jet at a meniscus of themelt formed against the seed. A portion of the melt is frozen to form asheet of the material that is horizontal on a surface of the melt. Thesheet is removed from the melt.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present disclosure, reference is madeto the accompanying drawings, which are incorporated herein by referenceand in which:

FIG. 1 is a cross-sectional side view of an embodiment of an apparatusthat separates a sheet from a melt;

FIG. 2 is a cross-sectional side view of a second embodiment of anapparatus that separates a sheet from a melt;

FIG. 3 is a cross-sectional side view of meniscus stabilization forLASS;

FIG. 4 is a cross-sectional side view of an embodiment of meniscusstabilization using gas impingement;

FIG. 5 is a cross-sectional diagram illustrating pressure distributionfrom a gas jet;

FIG. 6 is a cross-sectional diagram of an embodiment of meniscusstabilization using gas impingement with a spillway;

FIG. 7 is a cross-sectional side view of a first embodiment of a gas jetwith sheet formation;

FIG. 8 is a cross-sectional side view of a second embodiment of a gasjet with sheet formation;

FIG. 9 is a cross-sectional side view of a third embodiment of a gas jetwith sheet formation;

FIGS. 10A-D illustrate seeding enabled by gas jet stabilization; and

FIG. 11 is a cross-sectional view of an embodiment of a gas jet.

DETAILED DESCRIPTION

The embodiments of the apparatus and methods herein are described inconnection with solar cells. However, these also may be used to produce,for example, integrated circuits, flat panels, LEDs, or other substratesknown to those skilled in the art. Furthermore, while the melt isdescribed herein as being silicon, the melt may contain germanium,silicon and germanium, gallium, gallium nitride, other semiconductormaterials, or other materials known to those skilled in the art. Thus,the invention is not limited to the specific embodiments describedbelow.

FIG. 1 is a cross-sectional side view of an embodiment of an apparatusthat separates a sheet from a melt. The sheet-forming apparatus 21 has avessel 16. The vessel 16 may be, for example, tungsten, boron nitride,aluminum nitride, molybdenum, graphite, silicon carbide, or quartz. Thevessel 16 is configured to contain a melt 10. This melt 10 may besilicon. A sheet 13 will be formed on the melt 10. In one instance, thesheet 13 will at least partly float within the melt 10. While the sheet13 is illustrated in FIG. 1 as floating in the melt 10, the sheet 13 maybe at least partially submerged in the melt 10 or may float on top ofthe melt 10. The depth at which the sheet 13 is located is based partlyon the relative densities of the sheet 13 and melt 10. In one instance,only 10% of the sheet 13 protrudes from above the top of the melt 10.The melt 10 may circulate within the sheet-forming apparatus 21.

This vessel 16 defines at least one channel 17. This channel 17 isconfigured to hold the melt 10 and the melt 10 flows from a first point18 to a second point 19 of the channel 17. The melt 10 may flow due to,for example, a pressure difference, gravity, a pump, or other methods oftransport. The melt 10 then flows over the spillway 12. This spillway 12may be a ramp, a weir, a ledge, a small dam, or a corner and is notlimited to the embodiment illustrated in FIG. 1. The spillway 12 may beany shape that allows a sheet 13 to be separated from the melt 10.

In one particular embodiment, the vessel 16 may be maintained at atemperature slightly above approximately 1685 K. For silicon, 1685 Krepresents the freezing temperature or interface temperature. Bymaintaining the temperature of the vessel 16 to slightly above thefreezing temperature of the melt 10, the cooling plate 14 may functionusing radiation cooling to obtain the desired freezing rate of the sheet13 on or in the melt 10. The cooling plate 14 in this particularembodiment is composed of a single segment or section but also mayinclude multiple segments or sections. The bottom of the channel 17 maybe heated above the melting temperature of the melt 10 to create a smallvertical temperature gradient in the melt 10 at the interface to preventconstitutional supercooling or the formation of dendrites, or branchingprojections, on the sheet 13. However, the vessel 16 may be anytemperature above the melting temperature of the melt 10. This preventsthe melt 10 from solidifying on the vessel 16.

The sheet-forming apparatus 21 may be maintained at a temperatureslightly above the freezing temperature of the melt 10 by at leastpartially or totally enclosing the sheet-forming apparatus 21 within anenclosure. If the enclosure maintains the sheet-forming apparatus 21 ata temperature above the freezing temperature of the melt 10, the need toheat the sheet-forming apparatus 21 may be avoided or reduced andheaters in or around the enclosure may compensate for any heat loss.This enclosure may be isothermal with anisotropic conductivity. Inanother particular embodiment, the heaters are not disposed on or in theenclosure and are rather in the sheet-forming apparatus 21. In oneinstance, different regions of the vessel 16 may be heated to differenttemperatures by embedding heaters within the vessel 16 and usingmulti-zone temperature control.

The enclosure may control the environment where the sheet-formingapparatus 21 is disposed. In a specific embodiment, the enclosurecontains an inert gas. This inert gas may be maintained at above thefreezing temperature of the melt 10. The inert gas may reduce theaddition of solutes into the melt 10 that may cause constitutionalinstabilities during the formation of the sheet 13.

The cooling plate 14 allows heat extraction that enables the sheet 13 toform on the melt 10. The cooling plate 14 may cause the sheet 13 tofreeze on or in the melt 10 when the temperature of the cooling plate 14is lowered below the freezing temperature of the melt 10. This coolingplate 14 may use radiation cooling and may be fabricated of, forexample, graphite, quartz, or silicon carbide. Disturbances to the melt10 may be reduced while the sheet 13 forms to prevent imperfections inthe sheet 13. Cooling a sheet 13 on the surface of the melt 10 or asheet 13 that floats on the melt 10 allows the latent heat of fusion tobe removed slowly and over a large area while having a relatively largesheet 13 extraction rate.

After the sheet 13 is formed on the melt 10, the sheet 13 is separatedfrom the melt 10 using the spillway 12. The melt 10 flows from the firstpoint 18 to the second point 19 of the channel 17. The sheet 13 willflow with the melt 10. This transport of the sheet 13 may be acontinuous motion. In one instance, the sheet 13 may flow atapproximately the same speed that the melt 10 flows. Thus, the sheet 13may form and be transported while at rest with respect to the melt 10.The shape of the spillway 12 or orientation of the spillway 12 may bealtered to change the velocity profile of the melt 10 or sheet 13.

The melt 10 is separated from the sheet 13 at the spillway 12. In oneembodiment, the flow of the melt 10 transports the melt 10 over thespillway 12 and may, at least in part, transport the sheet 13 over thespillway 12. This may minimize or prevent breaking a sheet 13 because noexternal stress is applied to the sheet 13. Of course, the sheet 13 alsomay be pulled or have some external force applied. The melt 10 will flowover the spillway 12 away from the sheet 13 in this particularembodiment. Cooling may not be applied at the spillway 12 to preventthermal shock to the sheet 13. In one embodiment, the separation at thespillway 12 occurs in near-isothermal conditions. The sheet 13 may tendto go straight beyond the spillway 12 in one embodiment. This sheet 13may be supported after going over the spillway 12 in some instances toprevent breakage.

Of course, different cooling temperatures across the length of thecooling plate 14, different flow rates of the melt 10 or pull speeds ofthe sheet 13, the length of the various sections of the sheet-formingapparatus 21, or the timing within the sheet-forming apparatus 21 may beused for process control. If the melt 10 is silicon, the sheet 13 may bepolycrystalline or single crystal sheet using the sheet-formingapparatus 21. FIG. 1 is only one examples of a sheet-forming apparatusthat can form a sheet 13 from a melt 10. Other apparatuses or methods ofhorizontal sheet 13 growth are possible. The embodiments describedherein may be applied to any horizontal sheet 13 growth method orapparatus. Thus, the embodiments described herein are not limited solelyto the specific embodiment of FIG. 1. For example, FIG. 2 is across-sectional side view of a second embodiment of an apparatus thatseparates a sheet from a melt. In the sheet-forming apparatus 31, themelt 13 is contained in the vessel 16. A sheet 13 is pulled from themelt 10 after formation by the cooling plate 14. While horizontal inFIG. 2, the sheet 13 also may be at an angle with respect to the melt10. In the embodiments of FIGS. 1-2, the melt 10 may circulate aroundthe sheet-forming apparatus 21 or sheet-forming apparatus 31, such asaround the sides of the sheet-forming apparatus 21 or sheet-formingapparatus 31. Of course, the melt 10 also may be stationary during partor all of the sheet 13 formation process.

A meniscus interface forms when a liquid is in contact with a gas. Thisinterface follows the Young-Laplace Equation. In two dimensions, ittakes the form:

${\Delta\; P} = \frac{\sigma}{R}$$\frac{1}{R(x)} = \frac{\frac{\partial^{2}y}{\partial x^{2}}}{\left( {1 + \left( \frac{\partial y}{\partial x} \right)^{2}} \right)^{3/2}}$where ΔP is the difference in pressure across the interface, σ is thesurface tension of the liquid, and R is the radius of curvature of thesurface.

FIG. 3 is a cross-sectional side view of meniscus stabilization forLASS. In terms of the coordinate system of FIG. 3, the radius ofcurvature may be expressed in terms of the first and second derivativesof the line describing the meniscus. The difference in pressure acrossthe meniscus 27 in FIG. 3 is due to the hydrostatic pressure in theliquid of the melt 10 due to gravity (ρgy). Thus, the Young-LaplaceEquation becomes a second order differential equation:

$\frac{\partial^{2}y}{\partial x^{2}} = {{- \frac{1}{\sigma}}\left( {\rho\;{{gy}(x)}} \right)\left( {1 + \left( \frac{\partial y}{\partial x} \right)^{2}} \right)^{3/2}}$

The concave shape of the meniscus 27 illustrated in FIG. 3 is providedby the negative pressure (P₁) relative to the atmospheric pressure(P_(atmos)). This is formed by lifting up and angling the sheet 13 withrespect to the X-axis to allow the higher elevation of the sheet 13 atthe edge of the wall of the vessel 16 while the freezing front of thesheet 13 is lower at the melt 10 height. In FIG. 3, α represents thecontact angle between the melt 10 in the meniscus 27 and the sheet 13and θ represents the contact angle between the melt 10 in the meniscus27 and the vessel 16. For a melt 10 composed of silicon and a vessel 16composed of quartz, θ is approximately 87°. These angles may vary basedon the materials. While the melt 10 level may be at the x axis in FIG.3, the melt 10 level may be elsewhere relative to the vessel 16.

Gas jets may be used to stabilize the meniscus by increasing localpressure in the melt. For example, local pressure on the gas side of themeniscus may be increased. Meniscus stabilization using the embodimentsdescribed herein is independent of any melt flow so that crystalinitialization may occur before the melt begins to flow, whichsimplifies seeding in systems that use melt flow. The sheet may be grownhorizontally using the embodiments described herein, which eliminatesthe complicated balance of growth rate (i.e., heat removal) against pullspeed at an angle. Sheet growth may be upstream of the edge of thevessel where separation from the melt occurs.

FIG. 4 is a cross-sectional side view of an embodiment of meniscusstabilization using gas impingement. In this embodiment, the sheet 13 ispulled horizontally. The wall of the vessel 16 is below the level of thesurface of the melt 10, which in this instance is at the intersection ofthe x and y axes or where the sheet 13 is located. The melt 10 isprevented from spilling by the formation of the meniscus 27 under thesheet 13. Of course, while not illustrated, the melt 10 may stillcirculate over the edge of the vessel 16, such as using the spillway 12illustrated in FIG. 1, and still form a meniscus 27, as illustrated inFIG. 6. Turning back to FIG. 4, The pressure difference to maintain theconcave meniscus 27 is provided by the gas jet 22 under the meniscus 27,which is angled toward the meniscus 27 or edge of the vessel 16 asindicated by the arrow leaving the gas jet 22. In such an instance, theYoung-Laplace Equation takes the form:

$\frac{\partial^{2}y}{\partial x^{2}} = {\frac{1}{\sigma}\left( {{{- \rho}\;{{gy}(x)}} - {P_{jet}\left( {x,y} \right)}} \right)\left( {1 + \left( \frac{\partial y}{\partial x} \right)^{2}} \right)^{3/2}}$The Young-Laplace Equation in this case is a second order differentialequation that requires two boundary conditions. In the embodiments ofFIGS. 3-4, the meniscus 27 is pinned to the wall of the vessel 16, sothat its location is fixed at x=0. At the other end of the meniscuswhere it connects with the sheet 13, the meniscus 27 is not pinned andthe angle made with the sheet 13 is determined by the surface energybetween the solid, liquid, and the gas. For a solid silicon sheet 13 incontact with its melt 10, the contact angle α may be betweenapproximately 0° and 11°. If y₀ is specified at x=0, the location of thecontact point of the meniscus 27 with the sheet 13 and the initialcontact angle are determined by the solution to the differentialequations.

The magnitude of the pressure at the exit of the gas jet 22 depends onthe flow of gas and the width of the opening in the gas jet 22 thatallows the flow of gas. The opening may be, for example, a slit jet.This may be at least partly estimated using conservation of momentum. Soat the stagnation point where the gas bounces off the meniscus 27, thepressure would be:

$P = {{\frac{1}{2}\rho_{g}u_{g}^{2}} = {\frac{1}{2}{\rho_{g}\left( \frac{Q_{g}}{A} \right)}^{2}}}$where ρ_(g), u_(g), and Q_(g) are the gas density, velocity, and volumeflow rate, respectively. The following example calculates the flow ofargon needed to obtain a pressure of 40 Pa at the meniscus 27 through anopening in the gas jet 22 that is 0.5 mm in width. The density of theargon at the temperature of the melt 10, which is 1412° C. for silicon,is 0.32 kg/m³.

$u_{g} = {\left( \frac{{{2 \cdot 40}\mspace{14mu}{Pa}}\;}{{.32}\frac{kg}{m^{3}}} \right)^{1/2} = {15.8\frac{m}{s}}}$$\frac{Q_{g}}{L} = {{u_{g} \cdot {gap}} = {{15.8{\frac{m}{s} \cdot {.5}}\mspace{14mu}{mm}} = \frac{7.9\frac{l}{s}}{m}}}$for 20  cm  wide  ribbon  Q_(g) = 95  lpm

The pressure estimated here may only exist at the exit of the gas jet22.

P_(jet)(x, y) = P₀𝕖^(−(a(x − x₀)² + 2 b(y − y₀)²(x − x₀)² + c(y − y₀)²))$a = {\frac{\cos^{2}\theta}{2\;\sigma_{x}^{2}} + \frac{\sin^{2}\theta}{2\;\sigma_{y}^{2}}}$$b = {{- \frac{\sin\; 2\;\theta}{4\;\sigma_{x}^{2}}} + \frac{\sin\; 2\;\theta}{4\;\sigma_{y}^{2}}}$$c = {\frac{\sin^{2}\theta}{2\;\sigma_{x}^{2}} + \frac{\cos^{2}\theta}{2\;\sigma_{y}^{2}}}$The pressure may fall off axially and transversely. The pressuredistribution may be approximated as an elliptical Gaussian.

FIG. 5 is a cross-sectional diagram illustrating pressure distributionfrom a gas jet. This solves the equations above. In all cases, P₀=40 Pa,x₀=6 mm at the exit of the gas jet 22, y₀=−4 mm at the exit of the gasjet 22, σ_(x)=4 mm, σ_(y)=0.8 mm, and ψ=45°. σ_(x) and σ_(y) representthe elliptical distribution of pressure around the gas jet 22. Where themeniscus 27 is pinned to the vessel 16 affects the shape of the meniscus27. When α=11°, the meniscus 27 is pinned to the wall of the vessel 16at 1 mm below the sheet 13 and θ=17°. When α=11°, the meniscus 27 ispinned to the wall of the vessel 16 at 2 mm below the sheet 13 andθ=15.87°. When α=11°, the meniscus 27 is pinned to the wall of thevessel 16 at 2.5 mm below the sheet 13 and θ=10.58°. When α=0°, themeniscus 27 is pinned to the wall of the vessel 16 at 1 mm below thesheet 13 and θ=7.21°.

Thus, by using gas impingement, a stable meniscus 27 can be pinned tothe wall of the vessel 16 at least 2.5 mm below the sheet 13 with acontact angle of approximately 11°. Even if the contact angle were aslow as 0°, a stable meniscus 27 could still be maintained 1 mm below thewall of the vessel 16. The gas jet impingement also may compensate forany drag caused by viscous forces. The pressure of the gas jetimpingement can be configured to stabilize the meniscus 27 or assist inmaintaining the pinning of the meniscus 27 to the vessel 16.

FIG. 6 is a cross-sectional diagram of an embodiment of meniscusstabilization using gas impingement with a spillway. The melt 10 flowsover the spillway 12. A meniscus 27 forms with the melt 10 passing overthe spillway 12. Of course, other embodiments are possible.

FIG. 7 is a cross-sectional side view of a first embodiment of a gas jetwith sheet formation. This system has a gas jet 22 separate from thesupport table 23. The support table 23 may use jets of air or some otherfluid to support the sheet 13, but also could use rollers or some othermechanism. In this particular embodiment, two gas jets 22, 25 are used,though greater or fewer may be provided. The gas jet 22 under the sheet13 stabilizes the meniscus 27 and may adjust position and angle. The gasjet 25 above the sheet 13 balances any vertical component of impingementforces from the gas jet 22. The flow from the gas jet 22 and gas jet 25may be approximately equal in one instance, though other flows arepossible. Argon, another noble gas, another inert gas, or other speciesknown to those skilled in the art may be used with the gas jet 22 or gasjet 25. The vessel 16 may contain a feature or groove 24 that pins themeniscus 27 of the melt 10 and allows a large variation in contact anglewithout drips. While a surface of the vessel 16 without the feature orgroove 24 can have a meniscus 27 pinned to it, the angle of the meniscus27 before drips occur is limited. For example, this angle may beapproximately 87°. Adding the feature or groove 24 enable the meniscus27 to sag or have an angle of approximately 177° from the surface of thevessel 16 before drips occur.

FIG. 8 is a cross-sectional side view of a second embodiment of a gasjet with sheet formation. In this embodiment, the gas jet 22 isincorporated into the support table 23. In an alternate embodiment, agas jet above the sheet 13 may be provided to balance verticalimpingement forces as illustrated in FIG. 7.

FIG. 9 is a cross-sectional side view of a third embodiment of a gas jetwith sheet formation. The gas jet 22 is part of a pressure cell 26.There is higher pressure (P₂) within the pressure cell 26 due to thelimited conduction at the edges or seals. The gas in the pressure cell26 flows as indicated by the arrows. P₂ in this instance is greater thanatmospheric pressure (P_(atmos)). The top section of the pressure cell16 floats on gas bearings over the bottom section. The level of thebottom section may be configured to be matched to the level of the melt10. The gaps or seals between the pressure cell 26 and the sheet 13 maybe less than 0.5 mm in dimension in one instance. The meniscus 27 iscontained within this pressure cell 26 at least partly because of thegas jet 22 that is part of the bottom section of the pressure cell 26.

FIGS. 10A-D illustrate seeding enabled by gas jet stabilization.Embodiments disclosed herein stabilize the meniscus independent of flowwithin the vessel 16. Thus, crystal initiation may begin before the melt10 begins to flow and simplifies the sheet fabrication process. In FIG.10A, a seed wafer 28 is inserted. The seed wafer 28 may be, for example,an electronics-grade silicon wafer approximately 0.7 mm thick with thedesired crystal orientation. The level of the seed wafer 28 iscontrolled by having it ride above a support table 23 that controls thelevel of the seed wafer 28 against the level of the melt 10. The melt 10may form a mesa 29 or be above the edge of the wall of the vessel 16using the surface tension of, for example, the silicon in the melt 10.Thus, before the seed wafer 28 touches the melt 10, a convex meniscus isformed in the melt 10, as illustrated in FIG. 10A. The gas jets 22, 25may be used to stabilize this meniscus before the seed wafer 28 touchesthe melt 10. Of course, more or fewer gas jets are possible.

In FIG. 10B, the seed wafer 28 touches the melt 10 after moving in thedirection of the arrow. A meniscus 27 forms beneath the seed wafer 28.This meniscus 27, which may be concave, bridges the gap between the wallof the vessel 16 and the seed wafer 28. Beyond the width of the seedwafer 28, the convex meniscus 27 remains attached to the wall of thevessel 16. There may be a non-uniform meniscus at this transitionbetween the mesa 29 meniscus and the concave meniscus 27 beneath theseed wafer 28. This non-uniformity likely will not affect the thicknessuniformity or quality of the sheet 13 because the process occurs in anearly isothermal environment.

The seed wafer 28 is translated in FIG. 10C in the direction of thearrow. This translation may be caused by a roller or some othermechanism at an end of the seed wafer 28. The seed wafer 28 moves underthe cooling plate 14 opposite the direction that the seed wafer 28 wasinserted. The cooling plate 14 may be turned off initially or be at atemperature at or above the temperature of the melt 10. If the coolingplate 14 is located a certain distance upstream of the wall of thevessel 16 where the meniscus 27 is attached, effects of the meniscus 27may be minimized. As the cooling plate 14 is turned on, freezing isinitiated near the seed wafer 28. The seed wafer 28 pulling motionbegins and a sheet 13 is pulled out.

In FIG. 10D, the melt 10 begins flowing using, for example, a pump. Themelt 10 may go over a spillway in an alternate embodiment. The width ofthe sheet 13 may be increased as the melt 10 begins to flow. Thetemperature of the cooling plate 14 and speed of the melt 10 flow orsheet 13 movement may be adjusted to achieve the desired thickness inthe sheet 13. Thus, a steady-state process may be achieved.

Meniscus stabilization using gas jets has multiple advantages. It may beapplied to a horizontal sheet formation or horizontal ribbon growth(HRG) system and may be used to avoid LASS in one instance. The sheet 13may be pulled horizontally in one embodiment, thereby allowing thecrystal forming region to be upstream of the meniscus 27. This minimizesperturbations caused by a pulling mechanism from affecting the sheet 13while it forms. The melt 10 flow speed may be controlled independentlyof the speed of the sheet 13. This may enable a simpler seeding process.Furthermore, spills of the melt 10 may be reduced or prevented.

FIG. 11 is a cross-sectional view of an embodiment of a gas jet. The gasjet 22 has a plenum 29 and opening 30. Gas flows in the direction of thearrow. Having a larger plenum 29 than an opening 30 may ensure uniformpressure and flow across the dimensions of the opening 30. In oneembodiment the opening 30 has a width approximately equal to the widthof the sheet, such as the sheet 13 in FIG. 4. Of course, otherdimensions are possible.

In the embodiments disclosed herein, the gas jet 22 may direct gas at aparticular temperature. The gas may be heated to prevent the meniscusfrom freezing. The gas also may be cooled to prevent the sheet frombeing melted or to otherwise cool the sheet.

The present disclosure is not to be limited in scope by the specificembodiments described herein. Indeed, other various embodiments of andmodifications to the present disclosure, in addition to those describedherein, will be apparent to those of ordinary skill in the art from theforegoing description and accompanying drawings. Thus, such otherembodiments and modifications are intended to fall within the scope ofthe present disclosure. Furthermore, although the present disclosure hasbeen described herein in the context of a particular implementation in aparticular environment for a particular purpose, those of ordinary skillin the art will recognize that its usefulness is not limited thereto andthat the present disclosure may be beneficially implemented in anynumber of environments for any number of purposes. Accordingly, theclaims set forth below should be construed in view of the full breadthand spirit of the present disclosure as described herein.

What is claimed is:
 1. A sheet production apparatus comprising: a vesselconfigured to hold a melt of a material; a cooling plate disposedproximate said melt, said cooling plate configured to form a sheet ofsaid material that is horizontal on said melt proximate said coolingplate; a first gas jet configured to direct a gas toward a meniscus at ajuncture of a bottom surface of the sheet and an edge of said vessel;and a pressure cell that is separate from and downstream of the coolingplate in a direction in which the melt flows, the pressure cell definedby a top section including a ceiling and adjoining sidewalls disposedabove the sheet over the first gas jet and defining a gap of less than0.5 millimeters between a lower extent of the sidewalls and the sheet,the pressure cell further defined by a bottom section including a floordisposed below the sheet and integral with the first gas jet, whereinthe pressure cell encloses a gas at a higher pressure than a surroundingatmosphere and wherein the gas is allowed to escape the pressure cellthrough the gap.
 2. The apparatus of claim 1, further comprising a pumpassociated with the vessel and oriented to circulate said melt withinsaid vessel.
 3. The apparatus of claim 1, further comprising a secondgas jet configured to direct said gas toward said edge of said vessel,said second gas jet positioned opposite said first gas jet and integralwith the top section.
 4. The apparatus of claim 1, further comprising asupport table disposed adjacent said first gas jet, said support tableconfigured to support said sheet.
 5. The apparatus of claim 1, whereinsaid material is silicon or silicon and germanium.